The present invention relates to an organic electroluminescence element used as a light source or a back light in various image forming apparatus or an light emitting element used in an optical communication equipment or the like, and or to an image forming apparatus and a terminal unit using thereof.
An electroluminescence element is the so-called light emitting device utilizing electroluminescence of a solid fluorescent substance, and an inorganic electroluminescence element using an organic group material as a light emitter has come into a practical use until now, and has been aimed for development and application into some backlights in a liquid crystal display, a flat display or the like. However, a voltage required for allowing the inorganic electroluminescence element to emit light is high, that is, it is not less than 100 V, and it is difficult to emit blue light therefrom, thereby full color exhibition with three primary colors, that is, RGB, is difficult. Further, since the inorganic electroluminescence element includes a light emitter made of a material having a large refractive index, it can be greatly affected by total reflection at its interface or the like, and accordingly, the efficiency of extraction of light into the air with respect to actual light emission is extremely low, that is, 10 to 20%, and it is difficult to enhance the efficiency.
Meanwhile, studies as to luminescence elements made of organic materials have been attractively and variously executed for a long time. However, its luminous efficiency is extremely low, and accordingly, it has not yet been developed into studies direct for practical use.
However, W. Tang of Kodak Co. proposed, 1987, an organic electroluminescence element having a function separation type laminated structure in which an organic material is separated into two layers, that is, a hole transmit layer and a luminous layer, and it was found that the luminance which is not less than 1,000 cd/m2 could be obtained therefrom (Refer to Applied Physics Letter, vol. 51, 1987, page 913 or the like, by C. W. Tang and S. A. Vanslayke). Thereafter, this organic electroluminescence element has been remarkably attractive, and studies for organic luminescence elements having a similar function separation type laminated structure are now prosperously made. In particular, an increase in efficiency and a prolongation of the use life thereof, which are indispensable for practical utilization thereof, have been sufficiently studied. Thus, these years, a display or the like using an organic electroluminescence element has been materialized.
Next, explanation will be made of a configuration of a typical conventional organic electroluminescence element with reference to FIG. 25 which is a sectional view illustrating an essential portion of the conventional electroluminescence element and in which there are shown a substrate 1, an anode 2, a hole transport layer 3, a luminous layer 4, and a cathode 5.
Referring to FIG. 25, the organic electroluminescence element is composed of the anode 2 which is formed on the substrate 1 made of glass or the like in a sputtering process, a resistive heating evaporation process or the like, and which is formed of a transparent conductive film made of ITO or the like, and the hole transport layer 3 which is formed on the anode 2, similarly by the resistive heating evaporation process or the like, and which is made N,N-diphenyl-N, Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-diphenyl-4,4xe2x80x2-diamin (which will be abbreviated as PTD) or the like, the luminous layer 4 which is formed on the hole transport layer 3 by a resistive heating evaporation process or the like, and which is made of 8-hydroxyquinoline aluminum (which will be hereinbelow abbreviated as Alq3) or the like, and the cathode 5 which is formed on the luminous layer 4 by a resistive heating evaporation process or the like, and which is formed of a metal film having a film thickness of 100 to 300 nm.
When a d.c. voltage or a d.c. current applied to the electroluminescence element having the above-mentioned configuration, using the anode 2 as a positive electrode and the cathode 5 as a negative electrode, holes are injected from the anode 2 into the luminous layer 4 through the intermediary of the hole transport layer 3, and electrons are charged from the cathode 5 into the luminous layer 4. In the luminous layer 4, the holes and the electrons are recombined, and accordingly, excitons which are produced through the recombination are shifted from a normal state to an excited state so as to cause a luminous phenomenon.
Referring to FIG. 26 which is a graph exhibiting a relationship between an energizing time and a relative luminance in an organic electroluminescence element, there are shown variation in luminance among three kinds of initial luminance.
Referring to FIG. 27 which is a graph exhibiting a relationship between an energizing voltage and luminance in an organic electroluminescence element, as shown in FIG. 26, it is found, the higher the luminance, the shorter the energizing time or the service life. Further, as shown in FIG. 27, the energizing voltage has to be higher in order to enhance the luminance. Thus, in order to materialize an organic electroluminescence element capable of having a long service life and maintaining a high luminous function, it is important to lower the luminance and to enhance the luminous efficiency. However, in order to practically use the organic electroluminescence element, a sufficiently high degree of luminance is required, that is, it is ineffective to simply lower the luminance.
As mentioned above, the organic electroluminescence element has a correlation between its luminance and its service life. Thus, there has been demanded an organic electroluminescence which can fully satisfy both enhanced luminance and long service life.
In the above-mentioned organic electroluminescence element, light emitted from fluorescent substance in the luminous layer 4 is emitted omnidirectionally from the fluorescent substance as a center, and is then radiated into the atmospheric air by way of the hole transport layer 3, the anode 2 and the substrate 1. Alternatively, the light is once emitted in a direction reverse to a light extracting direction (a direction toward the substrate 1), then it is reflected by the cathode 5, and is then radiated into the atmospheric air by way of the luminous layer 4, the hole transport layer 5, the anode 2 and the substrate 1.
However, during the course of passing through interfaces between mediums, light which is incident at an angle greater than an angle with which an emergent angle of refracted waves becomes 90 deg., that is, at a critical angle, cannot transmit through one of the interfaces if the refractive index of a medium on the incident side is grater than that of a medium on the emergent side, and accordingly, the light is totally reflected so that it cannot extracted into the atmospheric air.
It is noted here that the relationship between a refraction angle of light and refractive indices of different mediums at the interface between mediums is determined under the Snell""s Law. In view of the Snell""s Law, in the case of transmission of light from a medium having a refractive index n1 into a medium having a refractive index n2, a relationship n1*sin xcex81=n2*sin xcex82 is obtained between an incident angle xcex81 and an emergent angle xcex82. Accordingly, if n1 greater than n2 is effected, the incident angle xcex81=sinxe2x88x921(n2/n1) with which xcex82=90 deg. can be obtained, has been well-known as a critical angle, and therefore, if the incident angle is greater than this value, the light is totally reflected at the interface between the mediums.
Thus, in an electroluminescence element in which light is isotropically radiated, light radiated at an angle grater than the critical angle repeats total reflection at the interface so that it is confined within the element, that is, it cannot be radiated into the atmospheric air.
Referring to FIG. 28 which is a schematic view illustrating a typical light paths in a section of an essential part of a conventional organic electroluminescence element, and in which like reference numerals are used to denote parts like to those which have been explained with reference to FIG. 25, light rays radiated from a light source 6 in the luminous layer 4 are totally reflected at interfaces, that is, an interface between the anode 2 and the substrate 1 (ITO/glass interface) and an interface between the substrate 1 and the atmospheric air (glass/air interface).
That is, light rays emitted from the luminous layer 4 are not emitted outside the element so as to apparently cause lowering of the efficiency of the organic luminescence element. In general, it has been known that a substantial part of radiated light rays obtained in the luminous layer 4 are confined within the element, and accordingly, only about 17 to 20% thereof is used as effective light lays (Refer to Advanced Material 6 (1994) 491).
Thus, a means for changing the emergent angle of light is provided in the substrate of the organic luminescence element, in order to aim at solving the above-mentioned problem.
For example, JP-B2-2,773,720 discloses such an invention that a lens structure is formed on the light extracting side of the substrate so as to enhance the efficiency of the electroluminescence element.
Further, JP-B2-2,991,183 discloses such an invention that diffraction grating or the like is formed at positions where the total reflection should be restrained so as to enhance the efficiency of extraction of light, and further, JP-A-9-129375 discloses such an invention that a surface on the light extraction side of the element causes irregular reflection or disordered reflection or refraction angles so as to enhance the efficiency of extraction of light.
Further, JP-A-10-189251 discloses means for changing light emitting angles, formed in a transparent substrate, and JP-A-10-308286 discloses a light reflecting layer is formed on the lower electrode side surface so as to enhance the efficiency of extraction of light.
However, the above-mentioned conventional electroluminescence elements have caused the following problems:
In the case of using the element in an image forming device such as a display unit, since the mesa structure has an inverted V-like shape structural component on the element side surface of the substrate, it is very difficult to form an electrode while preventing the effect of the mesa structure from lowering.
In the case of using the element in an image forming apparatus of an active matrix type, it is extremely difficult to form electrodes while the effect of the masa structure is maintained.
In the case of formation of the mesa structure on the substrate, since light emitted from the organic electroluminescence element is extracted after transmission at least through both mesa structure and substrate, the light causes a light loss corresponding to transmission through the substrate, and accordingly, the efficiency of light emission is lowered.
In the case of using the organic electroluminescence element having a substrate formed thereon with a mesa structure in an image forming device such as a display unit, since light emitted from an arbitrary pixel is extracted after transmission at least through the mesa structure and the substrate, light totally reflected comes to the other pixels by way of the substrate, and is then emitted into the atmospheric air from these pixels so as to cause the so-called stray light, resulting in disadvantages including lowering of the contrast thereof.
If the mesa structure and the organic electroluminescence element are formed, independent from each other, the positional alignment thereof during affixing therebetween is difficult, causing a slip in affixing, which causes deterioration of the visual performance thereof such as moirxc3xa9 effect. Further, it is likely to cause a disadvantage of peeling-off due to stress.
Although the efficiency of extraction of light is enhanced, enhancement of both brightness and use life has not yet been satisfied, and further, prolongation of use life has not yet come into effect.
In the case of using the element in an image forming device as a display unit composed of a group of minute pixels, the areas of openings and luminous portions become smaller in the pixel area, and accordingly, it is required to effect luminance with a high degree of brightness.
Further, it has not yet been made to aim at prolonging the use life thereof in view of its element structure.
Further, since the orientation of light varies, the visibility thereof is lowered, including lowering of the visual angle.
In the case of using the element in a full color display or the like, visual angles of respective colors are different from one another, causing color shift.
Since countermeasure for enhancing the efficiency of extraction of light is used in the substrate itself, relative restrictions are imposed to materials and processes for forming the element, and in particular, in the case of using the element incorporating the above-mentioned countermeasure for enhancing the efficiency of extraction of light in an image forming device such as a display unit, each of the pixels are very small, and accordingly, the degree of freedom as to the countermeasure for enhancing the efficiency of extraction of light is important.
In the case of using the element in an image forming device such as a display unit, if pixels therein correspond to lenses in the lens structure one for one, the positional alignment of the lenses is difficult, causing a slip in position so as to result in deterioration of visibility.
In the case of using the element in an image forming device such as a display unit, since the lens structure is present on the light extraction surface side of the substrate, it is difficult shorten the distance between the lens structure and the luminance portion.
In the case of forming the lens structure on the substrate, since light emitted from the organic electroluminescence element is extracted into the atmospheric air after transmission at least through both lens structure and substrate, a light loss is caused in view of the transmission through the substrate, resulting in deterioration of the efficiency of light emission.
In the case of using an organic electroluminescence element having a substrate formed thereon a lens structure in an image forming device such as a display unit, since light radiated from an arbitrary pixel is extracted into the atmospheric air after transmission at least through both lens structure and substrate, light totally reflected comes to the other pixels by way of the substrate before it is radiated into the atmospheric air, and accordingly, it causes the so-called stray light, resulting in a disadvantage of lowering the contrast thereof and the like.
In the case of the provision of a countermeasure for enhancing the efficiency of extraction of light, such as the formation of a lens structure or a mesa structure on the substrate side, it is difficult to freely design the orientation of light, possibly causing lowering of visibility such as a visual angle characteristic which can be exhibited by the organic electroluminescence element.
The present invention is devised in order to solve the above-mentioned problems inherent to the prior art, and accordingly, one object of the present invention is to provide an organic electroluminescence element which is excellent in visibility and which can maintain a light emitting function with a high degree of efficacy, also to provide an image forming apparatus which is excellent in visibility and which can maintain a light emitting function with a high degree of efficiency, and as well to provide a portable terminal unit having a light weight and a long operating time.
According to the present invention, there is provided an organic electroluminescence element including a substrate incorporating thereon at least an anode for hole injection, a luminance layer having a luminance zone and a cathode for electron injection, light radiated from the luminance layer is extracted from a surface opposed to the substrate, and an inverted V-like shape structural component having a height higher than that of the luminance layer is formed at least in a part of an element forming surface of the substrate.